The invention relates to electronic devices, and more particularly to coding, transmission, storage, and decoding/synthesis methods and circuitry as is useful in spread spectrum systems.
Code division multiple access (CDMA) has been extensively used in such applications as satellite and cellular communications. CDMA signals increase the spectrum required for the transmission of a particular data rate by modulating each information symbol with a spread spectrum code having a rate larger than the data rate. The same spreading code is used for each information symbol. Typically, the spreading code comprises a few tens or a few hundreds of elements, called chips. To decrease the correlations among spreading codes assigned to different users, and thereby reduce the interference among different users, the data stream after spreading is typically scrambled with a pseudo-noise (PN) code that is generated serially and cyclically and has a larger period than the spreading code. Examples of such CDMA signal spreading are the schemes used by the IS-95/CDMA2000 and 3GPP systems. With CDMA, the signals from all users simultaneously occupy the same frequency band. The receiver discriminates the multiple signals by exploiting the properties of the spreading and scrambling codes that are applied to the signal of each user. The receiver attempts to match in time the codes of the desired signal with a replica of those spreading and scrambling codes. Only then is the demodulation result meaningful; otherwise it appears noise-like. Thus, if the arriving signals have different codes or different code offsets, they can be discriminated at the receiver.
In the forward link (downlink) of cellular communication systems (the communications from base stations to mobile terminals) the wireless channel may introduce multipath propagation. Even if the signals transmitted by the base station are spread using orthogonal codes (Walsh codes), the multipath propagation will destroy the orthogonality and produce multiple-access interference (MAI). That is, a code may not be orthogonal to a delayed version of another code. Interference cancellation (IC) attempts to suppress the MAI by estimating and subtracting the interference from the received signal. Because the capacity of CDMA systems is MAI-limited, estimating and canceling the MAI will increase the capacity. Alternatively, IC can reduce the signal-to-noise ratio (SNR) required to achieve a specific symbol-error-rate (SER) or frame-error-rate (FER) value for a specific number of users, thereby increasing mobile terminal battery life and decreasing transmission power. The implementation alternatives for IC are the parallel IC (PIC) and the serial IC (SIC). With PIC, all users are simultaneously demodulated, a tentative decision is made for their information symbols, the MAI is regenerated and the process is iterated a number of times until all significant improvements in the SER performance are achieved. With SIC, a similar process is performed with the exception that a decision on the stronger user is first made, the interference from that user on the decision statistic of the next stronger user is subsequently removed and a decision for the next stronger user is made, etc. The main advantage of SIC relative to PIC is that the decisions for the stronger users are more reliable than the decisions for the weaker ones and therefore, the MAI removal of the stronger users is reliable and this benefits the weaker users. The main disadvantage of SIC relative to PIC is the decision delay associated with SIC because of its serial nature and the architecturally difficult reordering of the detection process each time the relative signal strengths change. There are also several modifications that can be applied to the IC process. For example, the decisions made at each IC stage can be hard or soft, clipped or threshold-based or unmodified, the estimated IC can be weighted to account for the bias that exists in the decisions depending on the IC stage and for the reliability of the individual decisions depending on the signal strength.